362 COORDINATION AND SENSATION 



Six other openings lead off from the vestibule at different 

 places. One of these enters the cochlea. The other five 

 open into 



The Semicircular Canals. These canals, three in num- 

 ber, pass through the bone in three different planes. One 

 extends in a horizontal direction and the other two verti- 

 cally, but each plane is at right angles to the other two. 

 Both ends of each canal connect with the vestibule, though 

 two of them join by a common opening. The inner mem- 

 branous labyrinth is continuous through each canal, and 

 is held in position by small strips of connective tissue. 



The purpose of the semicircular canals is not under- 

 stood. It is known, however, that they are not used in 

 hearing. On the other hand, there is evidence to the 

 effect that they act as equilibrium sense organs, exciting 

 sensations necessary for balancing the body. Their re- 

 moval or injury, while having no effect upon the hear- 

 ing, does interfere with the ability to keep the body in an 

 upright position. 



The Cochlea is the part of the internal ear directly con- 

 cerned in hearing. It consists of a coiled tube which makes 

 two and one half turns around a 

 central axis and bears a close resem- 

 blance to a snail shell (Figs. 151 and 

 152). It differs in plan from a snail 

 shell, however, in that its interior 

 space is divided into three distinct 

 l ?' I , 53 ' T . iagram channels, or canals. These lie side 



showing the divisions of 



cochlear canal. b Y Slde and are named, from their 



relations to other parts, the scala 



vestibula,-'ChQ. scala tympani, and the scala. media. Any 

 vertical section of the cochlea shows all three of these 

 channels (Fig. 153). 



